The effectiveness of peptide and small molecule drugs is often limited by the half-life of such drugs in the circulation, as well as difficulties in obtaining substantially constant plasma levels. For example, the incretin GLP-1 must be administered at relatively high doses to counter its short half-life in the circulation, and these high doses are associated with nausea, among other things. Murphy and Bloom, Nonpeptidic glucagon-like peptide 1 receptor agonists: A magic bullet for diabetes? PNAS 104 (3):689-690 (2007). Further, the peptide agent vasoactive intestinal peptide (VIP) exhibits a half-life, in some estimates, of less than one minute, making this agent impractical for pharmaceutical use. Domschke et al., Vasoactive intestinal peptide in man: pharmacokinetics, metabolic and circulatory effects, Gut 19:1049-1053 (1978); Henning and Sawmiller, Vasoactive intestinal peptide: cardiovascular effects, Cardiovascular Research 49:27-37 (2001). A short plasma half life for peptide drugs is often due to fast renal clearance as well as to enzymatic degradation during systemic circulation.
Strategies for improving the pharmacokinetics of peptide and small molecule drugs are in great demand.